1. Field
The present specification generally relates to conversion of grayscale to Color using an Intensity Based Colormap.
2. Technical Background
A grayscale image typically comprises 256 shades of gray (Hue=0), see FIG. 1, starting at black and increasing in Luminance to white.
Infrared thermography (IRT) and other forms of thermal imaging detect radiation in the infrared range of the electromagnetic spectrum and produce images of that radiation called thermograms. Since infrared radiation is emitted by all objects above absolute zero according to black body radiation law, thermography makes it possible to see one's environment with or without visible illumination. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds. For example, humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to military and other users of surveillance cameras.
The use of grayscale colormaps to delineate such data is ubiquitous. However, referring to FIG. 2, traditional colormapping which utilizes Hued maps of a single or variety of Colors fail because they lack either the differentiation necessary for human perception or assign a false Color to a pixel. In the latter case, the lack of perceptual ordering, associated with grayscale mapping, is discussed in Rainbow Colormap (Still) Considered Harmful (David Borland and Russell M. Taylor II—IEEE Computer Society—March/April 2007):                For all tasks that involve comparing relative values, the colormap should exhibit perceptual ordering . . . increasing Luminance from black to white is a strong perceptual cue that indicates values mapped to darker shades of gray are lower in value than values mapped to lighter shades of gray . . . . The rainbow colormap is certainly ordered—from a shorter to longer wavelength of light but it is not perceptually ordered . . . confusion results because greater-than and less-than relationships are not immediately evident and we must infer them through remembering (an error-prone task) or consulting a legend (a needless distraction for determining order).        
This quote identifies a problem in the industry. For instance, referring again to FIG. 2, a false Color spectrum (e.g., Jet) is colorful and has high contrast but it is not intuitive as blue represents the low end, red represents the high end. The Jet system sacrifices realism for high contrast.
Even a tailored false Color overlay on a grayscale image (e.g., colorizing specific hot spots above a certain temperature bar) does not meaningfully convey gradations in data to the end-user.
Traditional grayscale maps, however, also have inherent limitations even though the perceptual ordering of the grayscale colormap (with its increasing Luminance from black to white) lends well to the display of intensity-based imagery.
Referring again to FIG. 1, there are 256 levels of gray in the grayscale colormap. The downside to the grayscale colormap, however, is that the human eye can only see about one hundred different shades of a given Color (the same applies to a pink, bone, copper or other mono-hued colormaps). In general, black represents the low end of the spectrum and white represents the high end. The values in between are intuitively determined in accordance with their observed Luminance. But, the human eye cannot discern as much variety in the grayscale image as there actually is within the data. Thus, over half of the grayscale Colormap is useless when used for human inspection. Grayscale images (black-gray-white) have intensity (since the Hue/Saturation are the same=0). Color, however, offers greater opportunities for communicating data since it usually involves three dimensions: Hue, Saturation, and Luminance.
Therefore, there is a need in the art to show both more detail and gradation in the data to a human viewer by converting an image with an alternate, perceptually ordered colormap.